Merge branch 'cleanup/remove-openmp-dependency' into 'master'

[cleanup] Remove OpenMP dependency after upstream fix in opm-common/release/2020.04

See merge request dumux-repositories/dumux!2102

cmake/modules/DumuxMacros.cmake

@@ -14,7 +14,3 @@ find_package(PTScotch)
 find_package(PVPython)
 find_package(Valgrind)
 find_package(Quadmath)
-# The following is required for being able to depend on OPM.
-# Remove once a better solution has been found at
-# https://github.com/OPM/opm-common/issues/1751.
-find_package(OpenMP)